Background The presence of cyclic nucleotides (CNs) such as cGMP and cAMP in plant cells has unequivocally been demonstrated as have cyclase and cyclic nucleotide phosphodiesterase activity, two key enzymes of cAMP metabolism (figure 1;1). Accumulating evidence suggests crucial roles for both cAMP and cGMP in plant cellular homeostasis (e.g. 2;3;4) but it is largely unknown what the targets are for CNs in plants. For example, in animal cells the main targets for CN based signalling consist of CN-dependent protein kinases (PKA and PKG) that alter protein activity, but no such kinases have been found in plants.There is some biochemical evidence for cyclic nucleotide-responsive protein kinases in plants (1) but the molecular and functional identification of a cAMP- or cGMP-dependent protein kinase from a higher plant is still outstanding.

In animals, cAMP and cGMP signals lead to a change in expression of many genes, often via PKA-dependent phosphorylation of transcription factors such as CREB (cAMP response element-binding protein). There are indications that CREBs also exist in plants. Only a few examples of cAMP and cGMP-dependent regulation of plant gene expression exist. For example, CHS, the gene encoding chalcone synthase, is induced by cGMP in dark-adapted soybean cell cultures whereas defense-related genes PAL and PR-1 are induced by cGMP in tobacco. It is not known whether cyclic nucleotide-stimulated induction of genes in planta works through a mechanism analogous to CREB in animal systems.

Project aims The objective of this project is to identify targets of CN signalling in plants. Thus, we will change the cellular levels of cAMP and cGMP and use a microarray approach to find genes whose expression is altered. We will analyse the promoters of co-regulated genes to see if concensus sequences are present. In parallel, we will isolate knockout mutants in genes that show high degrees of homology to animal CREBs, PKAs etc. to see how such mutations affect plant physiology and gene expression.

Methods We will use whole genome microarray assays, bioinformatics analysis and phenotype (growth) characterisation of mutant plants

References:
(1) Newton et al (1999) New Phytol 143:427-455
(2) Ehsan et al (1998) FEBS letters 422:165-169
(3) Penson et al (1996) Plant Cell 8:2325-2333
(4) Moutinho et al (2001) PNAS (USA) 98: 10481-10486
(5) Maathuis & Sanders (2001) Plant Physiol 127:1617-1625
(6) Elphick et al (2001) Plant Cell Environ 24:733-740
(7) Zhu (2001) TIPS 6:66-71.